Apoptosis, or programmed cell death, typically occurs in the development and maintenance of healthy tissues in multicellular organisms. Apoptotic pathways are known to play a critical role in embryonic development, viral pathogenesis, cancer, autoimmune disorders, and neurodegenerative diseases, as well as other events. Alterations in an apoptotic response has been implicated in the development of cancer, autoimmune diseases, such as systemic lupus erythematosis and multiple sclerosis, and in viral infections, including those associated with herpes virus, poxvirus, and adenovirus.
Activated caspases, a class of cysteine proteases, are known to initiate apoptosis after they have been activated. In normal cells, the caspases are present as catalytically inactive zymogens. Inhibitors of apoptosis proteins (IAPs) are a family of proteins, which contain one to three baculovirus IAP repeat (BIR) domains, namely BIR1, BIR2, and BIR3, and may also contain a RING zinc finger domain at the C-terminus. The classical human IAPs, XIAP, HIAP1 (also referred to as cIAP2), and HIAP2 (cIAP1) each have three BIR domains, and a carboxy terminal RING zinc finger. Other IAPs, for example NAIP has three BIR domains (BIR1, BIR2 and BIR3), but no RING domain, whereas Livin and ILP2 have a single BIR domain and a RING domain. The prototype X chromosome linked inhibitor of apoptosis (XIAP) can not only inactivate the activated caspases by directly binding to caspases 3, 7, and 9 via the BIR2 and BIR3 domains, but can also remove caspases and the second mitochondrial activator of caspases (Smac) from the cytosol by the ubiquitylation-mediated proteasome pathway via the E3 ligase activity of a RING zinc finger domain. The BIR3 domain of XIAP binds and inhibits caspase-9, which is responsible for initiating the cascade in response to genotoxic damage and many other triggers. The linker-BIR2 domain of XIAP inhibits the activity of caspases-3 and -7, which are two downstream or effector caspases. The BIR domains have also been associated with the interactions of IAPs with tumor necrosis factor-associated factor (TRAFs)-1 and -2, and to TAB1. The IAPs thus function as a ‘constraint’ on the caspase cascade, thereby preventing or inhibiting active caspases. Because of their central role, the IAPs are capable of suppressing cell death from a wide variety of triggers, including chemotherapeutic drugs and irradiation.
Overexpression of one or more of the IAPs has been documented in most established cancer cell lines, as well as in primary tumor biopsy samples. Chromosome amplification of the 11q21-q23 region, which encompasses both HIAP1 and HIAP2, has been observed in a variety of malignancies, including medulloblastomas, renal cell carcinomas, glioblastomas, and gastric carcinomas. Thus, the IAPs may directly contribute to tumor progression and resistance to pharmaceutical intervention.
Progress in the cancer field has now led to a new paradigm in cancer biology wherein neoplasia is viewed as a failure to execute normal pathways of apoptosis. Normal cells receive continuous feedback from their environment through various intracellular and extracellular factors, and “commit suicide” if removed from this context. Cancer cells, however, gain the ability to ignore or bypass this regulation and continue inappropriate proliferation.
The X-ray crystallographic structure of XIAP BIR2 and BIR3 reveals a critical binding pocket and groove on the surface of each BIR domain. Two mammalian mitochondrial proteins, namely second mitochondria-derived activator of caspases (Smac) and Omi/Htra2, and four Drosophila proteins (Reaper, HID, Grim, and Sickle), which interfere with IAP function by binding to these sites on the BIR domain, have been identified. Each of these IAP inhibitors possesses a short amino-terminal tetrapeptide, AXPY or AVPI-like, sequence that fits into this binding pocket and disrupts protein/protein interactions such as IAP-caspase interactions. Although the overall folding of individual BIR domains is believed to be generally conserved, there are alterations in the amino acid sequences that form the binding pocket and groove that suggest that binding affinities might vary between each of the BIR domains.
Cancer therapies, including radiation therapy and chemotherapy, have traditionally been viewed as causing overwhelming cellular injury due to their lack of specificity. Therefore the need to improve the specificity of agents used to treat cancer, and indeed other proliferative disorders, is important because of the benefits in decreasing the side effects associated with administration of these agents.
A number of compounds have been disclosed that demonstrate down regulation of XIAP. The action of the compounds does not appear to be via direct interaction with XIAP. The down regulation of XIAP is likely a result of increased protein degradation.
A number of peptidic and non-peptidic compounds have been described, which bind XIAP BIR3 (Sun et al., Bioorg. Med. Chem. Let. 15 (2005) 793-797; Oost et al., J. Med. Chem., 2004, 47(18), 4417-4426; Park et al., Bioorg. Med. Chem. Lett. 15 (2005) 771-775; Franklin et al., Biochemistry, Vol. 42, No. 27, 2003, 8223-8231; Kip et al., Biochemistry 2002, 41, 7344-7349; Wu et al., Chemistry and Biology, Vol. 10, 759-767 (2003); Glover et al., Analytical Biochemistry, 320 (2003) 157-169); United States published patent application number 20020177557; and United States published patent application number 20040180828).
The aforesaid compounds while they appear to target the BIR3 domain of XIAP, may have limited bioavailability and therefore limited therapeutic application. Moreover, the compounds may not be selective against other IAPs and indeed other BIR domains, such as BIR2; this lack of specificity may lead to unexpected side effects.
Thus, IAP BIR domains continue to remain an attractive target for the discovery and development of novel therapeutic agents, especially for the treatment of proliferative disorders such as cancer.